This invention relates generally to transmissions and more particularly to hydromechanical transmissions utilizing planetary gear assemblies and variable displacement hydrostatic units.
In heavy duty on/off highway vehicles such as large trucks and off-highway vehicles such as construction and agricultural vehicles, full power and torque drive are often required at all operating speeds. Smooth, uninterrupted high torque and power flow during acceleration are desirable for optimum performance of the vehicle. Oftentimes, a break in the power flow to the vehicle wheels may cause the vehicle to stall, especially if adverse ground conditions and extreme loads exist. For example, a farm tractor or a bulldozer may become stalled by an instantaneous break in the power train, such as by declutching the power train to shift gears.
Power shift transmissions have come a long way in solving these drive train interruption problems, but are limited in range and lose appreciable output horsepower between gear ratio changes. Known automatic transmissions lack the efficiency available in manual shift transmissions. However, manual shift transmissions are somewhat limited because most are driven exclusively through mechanical gearing and do not provide maximum vehicle performance in off-highway terrain. Power shift transmissions used in conjunction with torque converters may be abused by excessive operation at or near stall conditions which may cause overheating. Efforts to provide adequate cooling can be expensive and results in high pump losses with lower overall efficiency.
Vehicle efficiency may be best expressed in terms of minimum fuel consumption. Maximum fuel economy for a vehicle is obtained when the engine, transmission, and drive train are operating at peak efficiency. Optimum overall efficiency may be obtained by operating at the best engine speed for each road load condition. This is particularly important for the heavy duty line haul vehicle applications where a considerable amount of miles are logged on each vehicle per year.
Collateral uses for hydromechanical transmissions have been suggested, such as dynamic braking and regenerative starting of a vehicle by hydraulic power. These features are especially desirable for heavy duty vehicles such as on/off highway trucks, but heretofore known transmissions have not been able to provide these features without the addition of several expensive and complicated accessories which would not normally be included on the vehicle.
Power take-off (PTO) devices are also common in several types of heavy duty vehicles such as cement mixers, refuse collectors, etc. These PTO's can be either mechanically or hydraulically powered. Hydraulic PTO's can be very complex and expensive to install. They must produce a constant source of high pressure hydraulic fluid to be most effective. In previously known hydromechanical transmissions the fluid manifold includes fluid passageways which may alternatingly carry high and then low pressure fluid during operation. This causes problems because the associated hydraulic PTO pump or motor will correspondingly be driven forward and backward. Still other types of transmissions may provide adequate fluid pressure only during specific modes of operation, such as when the transmission is in neutral.
A further objection to previous known hydromechanical transmissions is that the hydraulic horsepower requirements as well as the hydrostatic unit operating speeds and pressures are quite high which results in loss of efficiency, high heat rejection, and objectionable noise levels.